LONDON — The prospect of an imminent Western military strike on Syrian government targets appeared to encounter a delay on Wednesday when Britain signaled it would first await the findings of a United Nations inquiry into the suspected use of chemical weapons in an attack that killed hundreds near Damascus last week, and then hold a separate parliamentary vote, which could be days away.

Prime Minister David Cameron of Britain, who runs a coalition government, is facing political difficulties from legislators mindful of the experience in Iraq, when assurances from Prime Minister Tony Blair and President George W. Bush that Saddam Hussein had weapons of mass destruction proved inaccurate and a false pretext for war.

Mr. Cameron bowed on Wednesday to pressure from the opposition Labour Party and to some within his own coalition who want to allow United Nations weapons inspectors a chance to report their findings and for the United Nations Security Council to make one more effort to give a more solid legal backing to military action against Damascus.

As Mr. Cameron ran into difficulties, the Syrian government, which has denied accusations by a range of Western and Arab countries that it used chemical weapons in the Aug. 21 attack, moved abruptly to prolong the inspectors’ visit, announcing that it had evidence of three previously unreported chemical weapons assaults that it said had been carried out by insurgents and should be investigated by the inspectors.

If they look into those accusations, the inspectors could remain in Syria well past this weekend, beyond their original mandate.

The developments slowed the momentum the United States and Britain had been building for military intervention in the Syrian conflict, which began more than two years ago as a popular uprising against President Bashar al-Assad and has since become a civil war that has left more than 100,000 people dead and destabilized the Middle East.

The American and British governments have said that the evidence is already persuasive that Mr. Assad’s forces used chemical munitions on civilians in the Damascus suburb of Ghouta last week, committing what the Obama administration has called a moral atrocity that cannot go unanswered.

The United States could still act without Britain’s support, but the Obama administration has actively sought to build a consensus for a military strike, and Britain is America’s closest ally. While expectations had been building that a strike could happen by the weekend, another few days may make no difference to what has been advertised as a short, sharp punishment for the use of chemical weapons, not an effort to oust Mr. Assad.

The British signal that it would not rush to military action came late Wednesday when Mr. Cameron’s government, aware of the sensitivities created by the legacy of the run-up to the Iraq war a decade ago, said unexpectedly in a motion to be voted on by Parliament on Thursday that a separate vote on military action would be required. That vote may not take place until next week.

The text of the motion states that “a United Nations process must be followed as far as possible to ensure the maximum legitimacy for any such action,” and that the secretary general “should ensure a briefing to the United Nations Security Council immediately upon the completion of the team’s initial mission.”

Mr. Cameron’s pullback came as Britain moved to introduce a Security Council resolution that would authorize military action in Syria — a measure that Russia, the Syrian government’s most important backer, quickly signaled it would block, as it has done several times.

After an informal meeting among the five permanent Council members at United Nations headquarters in New York, no further action on the resolution was taken. “This isn’t going anywhere,” a Western diplomat said.

The Russians argued that it was premature to even talk about such a resolution while United Nations inspectors were on the ground in Syria.

Syria’s ambassador to the United Nations, Bashar Jaafari, added a new level of complexity to the issue, announcing that he had submitted evidence of the three new instances of chemical weapons use in Syria, which he asserted had been carried out by armed terrorist groups, the government’s blanket term for Syrian opposition forces.

Mr. Jaafari said they occurred on Aug. 22, 24 and 25, and were also in the Damascus suburbs. He said Syrian soldiers were the targets. The ambassador did not explain why he waited to come forth with the allegations.

“The Syrian government is requesting the secretary general to immediately instruct the investigation team operating in Damascus to investigate immediately these three heinous crimes,” the ambassador said.

Mr. Jaafari repeated the Syrian government’s denials that it had ever used chemical weapons in the conflict and said the accusations were a conspiracy by Western nations acting on Israel’s behalf. He rejected assertions by the United States, Britain and other Western allies that there was persuasive evidence of Syrian government culpability in the use of the banned weapons.

“We are not warmongers,” he told reporters outside the Security Council chambers. “We are a peaceful nation seeking stability.”

In Washington, the State Department spokeswoman, Marie Harf, suggested that the failure of Britain’s Security Council resolution to move forward on Wednesday had been expected.

“All previous attempts to get the Security Council to act on Syria have been blocked, and we cannot allow diplomatic paralysis to be a shield for the perpetrators of these crimes,” she said. “We do not believe that the Syrian regime should be able to hide behind the fact that the Russians continue to block action on Syria at the U.N.”

Asked if the United States would await the findings of the United Nations inspectors, Ms. Harf repeated the administration’s assertions that their work was too late to be credible because Syrian government forces had repeatedly shelled the attack sites, compromising evidence-gathering efforts.

“We’re going to make our own decisions on our own timelines about our response,” she said. “Obviously, we will continue consultations with our international partners around the world, but we are making decisions based on our own timeline.”

Earlier Wednesday, Russian officials continued to warn against international intervention. The foreign minister, Sergey V. Lavrov, spoke by telephone with the United Nations special envoy to Syria, Lakhdar Brahimi, and warned that an attack would “only lead to the further destabilization of the situation in the country and the region,” according to a statement posted by the Foreign Ministry.

For his part, Mr. Brahimi told reporters in Geneva on Wednesday that international law required Security Council approval for any military action in Syria. Mr. Brahimi also said the United States and Britain had yet to share what they said was evidence that would establish that Mr. Assad’s government had used chemical weapons.

In a further sign of mounting tensions, Russia’s Emergency Services Ministry said it was evacuating more Russians and citizens from other former Soviet republics from Syria, where Moscow maintains a naval base and where thousands of its citizens live after decades as the main international sponsor of the government in Damascus.

Stephen Castle and Steven Erlanger reported from London, and Rick Gladstone from New York. Reporting was contributed by Michael R. Gordon from Washington, Alan Cowell from London, Steven Lee Myers from Moscow, Nick Cumming-Bruce from Geneva and Marlise Simons from The Hague.

A version of this article appears in print on , on Page A11 of the New York edition with the headline: Britain to Wait on U.N. Weapons Report and Parliament Vote Before Syria Strikes. Order Reprints | Today’s Paper | Subscribe